1. Technical Field
The present invention generally relates to a powder coating apparatus and method, and more particularly, to such a powder coating apparatus and method for spraying charged powder coating material on to an object to be coated so as to apply it to the object while using static electricity.
2. Description of the Related Art
Attention has been focused on electrostatic powder coating as a painting or coating method of the environment-friendly and pollution-free type without using any solvent from the viewpoint of environmental protection. In this electrostatic powder coating, powder coating material is supplied from a paint tank through an injector to a spray gun, where it is injected or sprayed, together with a carrier air stream, to an object to be coated from a nozzle opening formed at a tip end of the spray gun. At this time, a high voltage is impressed upon a corona electrode which is provided at the tip end of the spray gun with the object to be coated being grounded, so that a corona discharge is generated from the electrode of the spray gun toward the object to be coated. As a result, when the powder coating material injected from the nozzle opening passes through the neighborhood of the electrode, it is charged through its collision against ions generated by the corona discharge. The powder coating material thus charged is deposited on the surface of the object to be coated under the influence of the carrier air stream and electric forces generated along the electric lines of force.
However, when the corona discharge is continuously carried out, the generation of the corona discharge may be suppressed by a space charge of negative ions developed by the corona discharge itself, thus resulting in difficulty in providing a uniform corona discharge from the corona electrode. As a consequence, there might be a fear that the efficiency of coating to the object to be coated is reduced.
In addition, there might also arise another problem as stated below. That is, the surface potential of a coating film on the object to be coated increases gradually due to the coating of the charged powder coating material, there would take place dielectric breakdown between the surface of the object to be coated and the surface of the coating film. Therefore, the gas present therearound is ionized to release positive ions, so that there could be developed a so-called back ionization in which the negative ions generated by the corona discharge are neutralized by the positive ions, thus resulting in reduction in the quality of the coating film.